objc_autoreleasePoolPop() can throw if a -dealloc does.

Model it as throwing so that the exception can be caught.

This is generally not expected to have significant code-size
impact because the contents of the @autoreleasepool block
are very likely to contain a call, very likely at the same
cleanup level as the @autoreleasepool itself.

rdar://13660038

llvm-svn: 179630